1. Field of the Invention
The present invention relates to a method and an apparatus for controlling deformable actuators such as cantilever beam micromirror elements of a micromirror array, and, in particular, relates to an improved addressing scheme for multiple micromechanical actuators that reduces or even eliminates the negative effect of imprinting on the operation of the actuators.
2. Description of the Related Art
Imprinting is a cumulative material effect that limits the performance of micromechanical actuators and occurs, for example, for actuators made of aluminum and aluminum based alloys. The imprinting effect manifests itself by a gradually increased change of the actuator position when all parameters are kept constant in the deflected state. It further manifests itself by curing out when the actuator is left in the non-deflected state for a sufficient amount of time. Both, the build-up and the curing out are non-linear effects. The resulting position at a given time thus depends not only on the addressing force at that time, but also on the history of deflection or deformation of this particular actuator element. Therefore, the response of the actuator becomes inaccurate unless the actuator is used as a two-state-actuator switching from one of two states to the other with the states being defined by stops.
One example where imprinting has a negative effect on the behaviour of multiple actuators are cantilever beam micromirrors as used in spatial light modulators (SLM). FIG. 5 schematically shows two exemplary micromirror elements 50a and 50b of a SLM micromirror array. Each of the micromirror elements 50a and 50b comprises an actuator element 52a and an address electrode 54a and 54b, the actuator element 52a and 52b and the address electrode 54a and 54b facing each other over a gap 56a and 56b. Each actuator element 52a, 52b is supported by a deflectable portion 58a and 58b so as to be deformable or deflectable into the gap 56a and 56b, respectively, the deflected state of the actuator elements 52a′, 52b′ being shown by dashed lines, and the non-deflected state being sown by solid lines. Each actuator element 52a and 52b serves as an electrode and is connected to a driver terminal 60 to which a common driver signal is applied. Each address electrode 54a and 54b is connected to a different address terminal 62a and 62b to which one of two address signals or voltages is applied in order to cause the actuator elements to assume one of two deformation states. More particularly, due to the huge number of elements in a micromirror array, the deformation state of each micromirror element 50a and 50b is adjusted serially before each. SLM flash. Data defining the deflection states of each micromirror element 50a and 50b is written to memory cells each corresponding to one of the micromirror elements 50a and 50b, by a demultiplexer during a so called programming time. After programming the data stored in the memory cells is applied to address terminals 62a and 62b of the micromirror elements 50a and 50b thereby changing the deformation states of the micromirror elements 50a and 50b or leaving same unchanged.
In order to achieve a high reflectivity of the micromirror elements 50a and 50b, they are preferably made of aluminium so that imprinting occurs.
The operation of the above micromirror array can be controlled in different ways. The operation of the above actuators of FIG. 5 according to a simple realisation and the occurrence of imprinting as well as the problems related thereto are discussed with reference to FIG. 6. FIG. 6 shows two graphs being arranged in registration, one on top of the other. The X axis of both graphs represent the time in arbitrary units. The Y axis of the upper graph represents voltage U in arbitrary units. The Y axis of the lower graph represents force F or the amount of deformation d in arbitrary units. In the upper graph, an example for a signal course or waveform of the potential or signal applied to the addressing electrode 54b of the actuator element 50b is shown by a narrowly dashed line, a signal course or waveform of the potential applied to the addressing electrode 54a of the actuator element 50a is shown by a dotted line, and a signal course of the potential applied to the driving terminal 60 is shown by a widely dashed line.
According to FIG. 6 the deflection states of the micromirror elements 50a and 50b are controlled directly by the addressing signals which are, as discussed above, set to the data stored in corresponding memory cells after each programming cycle thereby changing the deflection state of each micromirror element 50a and 50b or not. As can be seen, in the example of FIG. 6, the potential being applied to the addressing electrodes 54a and 54b is constant but different during the time shown in FIG. 6. In particular, the potential of addressing electrode 54a is equal to a first potential or voltage level U1 corresponding to a first deflection state, and the potential of addressing electrode 54b is equal to a second potential U2 corresponding to a second deflection state. No change in the deflection state has occurred. The potential applied to the driver terminal 60 is fixed and is equal to the potential of the addressing electrode 54a. 
In the lower graph, the force acting on and the deformation experienced by the actuator elements 52a and 52b, respectively, is shown, wherein the narrowly dashed line corresponds to actuator element 52b while the dotted line corresponds to actuator element 52a. As can be seen from FIG. 6, the actuation principle of the micromirror elements 50a and 50b is based on an electrostatic attraction between the respective addressing electrode 54a and 54b and the actuator element 52a and 52b, respectively. If the addressing electrode 54a, 54b, is set to the first potential U1 no force is acting whereas, when setting the addressing electrode 54a, 54b, to a second potential, the electrostatic attraction deflects or deforms the actuator element 52a and 52b. During operation of the micromirror array, the different actuator elements 52a and 52b develop a different amount of imprinting due to their different deformation states they assume during operation. This results in a different behaviour of these actuator elements in the further operation of these actuator elements resulting in an inaccurate light modulation.
The key for improved performance is that the accurate actuator position is only needed during a very short event. For example, when a micromirror array having the micromirror elements of FIG. 5 is used together with a laser beam in, for example, maskless optical direct writing, the position of the accurate actuators is only important during a laser flash indicated in FIG. 6 by reference number 70 and 72, respectively. The repetition period of this event might be long compared to this short event. For example, in case of a micromirror array being used for printing, most of the time is used for programming the desired control values to the multitude of actuator elements. These control values are, thereafter, as described above, serially written to memory cells and then applied to the addressing electrodes of the micromirror elements. In case of the simple addressing scheme of FIG. 6 in which the deformation state of the micromirror elements is directly controlled by the potentials applied to the addressing electrodes which are changed only after each programming cycle this leads to a bad ratio between the time (event times) in which the deflection of the mircomirror elements is necessary and the time (non-event times) in which no deflection is needed.
An addressing scheme that avoids unnecessary deformation of the actuator elements during non-event times and thus reduces the mean deflection of the actuators is described in the Swedish Patent Application SE01003667 with the title “Addressing Method and Apparatus using the same”. In FIG. 7, one version of such an improved addressing scheme is shown, wherein this version is described in the following with regard to the micromirror elements of FIG. 5. Similarly to FIG. 6, two graphs are shown, the upper one of which has an Y axis representing voltage U in arbitrary units, the lower one of which has an Y axis representing force F or deformation d in arbitrary units. The X axis are in registration and represent time t in arbitrary units. In the upper graph, exemplary signal courses for the potentials of the addressing electrode 54b of the micromirror element 50b, the actuator elements 52a and 52b and the addressing electrode 54a of the micromirror element 50a are shown by narrowly dashed, widely dashed and dotted lines, respectively.
Deviating from the simple addressing scheme of FIG. 6, according to the addressing scheme of FIG. 7 the deflection state of each micromirror element is firstly defined in a programming cycle during which each element is not deflected and then triggered by varying the common driver signal from a first to a second potential during the event times thereby deflecting the micromirror elements as defined by the addressing signals being applied. Thus, the time the micromirror elements experience deflection is limited to the event times by varying the common driving signal thereby acting directly and concurrently on all micromirror elements without substantially disturbing the potential difference between the addressing electrodes of the micromirror elements as defined by the preprogrammed control values. Thus, the common driver signal is used as a trigger for the deflection of the micromirror elements during event times.
As can be seen in FIG. 7, the potential exemplarily being applied to the addressing electrodes 54a and 54b is constant. In particular, the potential of addressing electrode 54a is equal to the first potential U1, and the potential of addressing electrode 54b is equal to a second potential U2. The potential applied to the driver terminal 60 is equal to a third potential U3 that is a mean potential between potentials U1 and U2, during non-event times 78. During event times, the potential applied to the driver terminal 60 is set to the potential U1.
In the lower graph, resulting forces acting on, or deformations experienced by, actuator elements 52b, 52a are shown by dashed and dotted lines, respectively. In non-event times 78, the potential difference between the addressing electrodes 54a and 54b and the actuator elements 54a and 54b, respectively, is only half compared to the addressing scheme according to FIG. 6, thus creating only one quarter of the force. Moreover, the force and deflection during non-event times 78 is the same for the actuated actuator element 52b, the addressing electrode voltage of which equals the second potential U2, and the non-actuated actuator element 52a, the addressing electrode voltage of which equals the first potential U1. Only during the short event times indicated at 80 and 82, the actuator elements 52a and 52b experience different amount of deflection. In particular, during event times 80, 82, the actuator potential set to the second potential U2 creates the full force for the actuated element 52b, whereas the actuator potential set to the first potential U1 creates no force for the non-actuated element 52a, thereby creating the desired difference of the actuator element positions. Thus, the mean load of the actuator elements is greatly reduced compared to the addressing scheme of FIG. 6. In particular, the difference between deflected and non-deflected actuator elements is cancelled for the biggest part of the operation time, the programming time 78.
However, during the event time, the pixels of the micromirror array are deflected very differently, which is, of course, on the one hand, the desired effect of the actuators but has, on the other hand, negative effects on the addressing accuracy.